Transcript What is wrong with this account?

Comments concerning the detection of
threats based on radioactive material
Dr. Rick B. Spielman
ABQ Technologies, Inc.
Albuquerque, NM 87123
What is wrong with this account?
What is very good with this account?
• Weapons-Grade Uranium Seized in Bulgaria -- In May 1999,
Bulgarian customs officials at the Ruse border crossing in Bulgaria
discovered 10 grams of highly enriched uranium (235U) inside a lead
"pig" concealed in an air compressor in the trunk of a car. The 35year-old Turkish driver was transporting his nuclear cargo from
Turkey to Moldova. It is believed the smuggler was offering this small
shipment of 235U as a "sample" to buyers in Moldova in advance of a
larger shipment of the same materials. The Bulgarian customs officer
who found the 235U had just received counter-proliferation training
from Customs and Border Protection. His supervisor had been
trained by Customs and Border Protection at RADACAD. The
Bulgarian lab director who examined and identified the materials had
also received training from CBP and Border Protection.
What are the limitations of existing
detector systems?
• Testimony to Congress describes…
• CBP: Radiation isotope identification device Handheld tool used to scan vehicles for materials that
emit radiation.
• CBP: Radiation-detection pagers: Worn by border
agents to pick up radiation emissions.
• Is Congress getting detailed information?
The easy problems
• Most radiation detection systems are designed
around high activity sources
–
–
60Co
at 1.25 MeV (1.32 MeV also)
137Cs at 611 keV
• Even small amounts of these materials are
detectable by relatively small, simple detectors.
• These radiation sources are difficult to shield.
The hard problems
• It is important for all agencies, responsible for detecting a
potential weapon of mass destruction, to know how hard it
is to detect nuclear weapons.
–
–
235U
bombs1 emit little detectable gamma rays
239Pu bombs (shielded) essentially emit no detectable g rays
• In these cases SIZE MATTERS and DISTANCE
MATTERS for radiation detectors.
– Hand-held detectors are totally ineffective.
1Lamarsh,
Basic Nuclear Engineering
An example is in order
• You are a Coast Guard officer responsible for radiation
monitoring of in-bound shipping.
– What can you see with a hand-held g-ray detector (4 cm2) at 200 m
distance? Attenuation by 200 m of air is 81%.
• Given a dirty bomb composed of 1 kg (4.2X1016 Bq) of
60Co shielded with 1 inch of lead (81% attenuation)
– This source will generate ~106 g/sec at the detector, which is an
easily detectable amount. (Ignoring self absorption.)
– On the ship this source is a serious health hazard.
• Given a nuclear weapon shielded with 1 cm of lead….
– NOTHING!
A comment about dirty bombs
• For a dirty bomb to have physical impact it must
have significant quantities of high activity
material.
– We typically are talking about 137Cs and 60Co.
– These are hard to shield and easy to detect with almost
any g-ray detector.
• If the amount of radioactive material is small
enough or well-shielded it probably can slip
through any detection system but it will have little
threat by itself.
It is a mistake to assume that terrorists
are stupid
• No terrorist is going to spend the time and money
needed to acquire or build a nuclear weapon and…
– then ship it into the US without any precautions.
• No terrorist is going to waste time shipping in
easy-to-detect radioactive material when...
– such material is readily available in the US.
• Today, Customs and Coast Guard personnel will
only detect radioactive material from stupid
terrorists.
A thorough cargo inspection system
must use multiple detectors
• Large, passive x-ray/g-ray detectors are a good
start.
• X-ray imaging systems are a big help in detecting
shielded radiation sources or nuclear weapons.
• Pulsed neutron sources will easily detect near
critical masses of fissile material.
– Personnel hazards are always a concern.
Bottom line: There is no single silver
bullet for detecting radioactive material
• You need to have good equipment
– Again SIZE and DISTANCE matter.
• Training, training, more training
– If you don’t understand the problem you will not use
your equipment effectively.
• You must understand the fundamental limitations
of your equipment.
– Know what you can detect and what you cannot and
why!
Why is a uranium-based nuclear
weapon hard to detect?
•
235U
weakly emits 185-keV x rays
– Easy to shield (0.5” lead transmits 5X10-8)
•
232U
235U
is hard to detect because of its low concentration in
– Low levels of 2614 keV g rays
• 10 kg of 235U contains 10-6 g of 232U
– This is 2.5X1015 232U atoms or 8X105 Bq.
• A U weapon emits 2.9X105 2614-keV g rays per second.
• Even the best detector with the best software still needs
~ 1 g -ray per second to overcome background.
• Ignoring any shielding - a 150-cm2 detector would have to
be less than 18 m away to detect a weapon
Why is a plutonium-based nuclear
weapon hard to detect?
•
239Pu
emits lower energy x rays.
– 51.6, 56.8, 98.7, 129, 375, & 414 keV.
• Low energy x rays are much easier to shield.
– Deliberate shielding can make Pu invisible.
• With 3 inches of shielding - a 150 cm2 detector
would have to be less than 7.5 cm away to detect
Pu 414-keV x rays.